The Americanized Yiddish word lox apparently comes from the old Yiddish laks, which in turn comes directly from the Germanlachs (which means salmon). In Swedish, salmon is lax - this in turn is the root of the term gravlax.

In point of fact, modern American regular lox is not smoked salmon. It is salmon which has been cured in a brine mixture of sugar, salt and spices. Generally, it will consist of salmon fillet; 'belly lox' is made from meat to either side of the fish's belly, which typically contains more fat. "Novy", "Nova Scotia" or "smoked salmon" is salmon which has been cured (usually with less salt) and then briefly cold-smoked to impart a smoky taste (as opposed to being smoked for preservative effect). Gravlax (like Scottish-style salmon) is dry cured, with salt and spices and other dry ingredients like juniper berry rubbed directly onto the fish without a brine solution. In the case of gravlax, it is then pressed to imbue the flavors and to remove as much moisture from the fish as possible.

Originally, of course, salmon was salt-cured as a preservative method. Today, however you take your salmon, it goes down a treat on a bagel with a schmear. I'm partial to onion bagels myself, from New York City of course. It's the water, you can't get good bagels anywhere else, even though the goyim try...